THE NUMBER OF HIGH-MASS GAMMA-RAY BINARIES (HMGBS) HAS BEEN INCREASING IN RECENT YEARS YET WE ARE STILL FAR FROM UNDERSTANDING THE NATURE OF THESE SYSTEMS. IN ALL BUT ONE CASE WE STILL CANNOT DIFFERENTIATE BETWEEN A NEUTRON STAR (NS) AND A BLACK HOLE (BH). WE PROPOSE NUSTAR OBSERVATION OF THE GAMMA-RAY BINARY LS 5039 OVER ENTIRE ORBITAL PERIOD TO OBTAIN A COMPLETE SPECTRAL AND TEMPORAL PORTRAIT OF THIS ARCHETYPAL HMGB. THE HIGH-S/N SPECTRAL AND HIGH-RESOLUTION TIMING DATA WILL ALLOW US TO SEARCH FOR NS OR BH SIGNATURES IN HARD X-RAYS.

George Washington University (The)

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